This application claims the benefit of Japanese Patent Application No. 2004-188728 filed in Japan on Jun. 25, 2004, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an in-vehicle antenna apparatus mounted on a windowpane of a vehicle, such as an automobile, and particularly, to an in-vehicle antenna apparatus provided with a patch-like radiation conductor disposed on an inner surface of a windowpane.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2002-252520 (p. 3 to p. 5, FIG. 1) discloses an example of a conventional in-vehicle antenna apparatus provided with a radiation conductor and a ground conductor disposed on an inner surface of rear glass or front glass of a vehicle. The radiation conductor and the ground conductor are connected to a circuit substrate via a coaxial cable such that the in-vehicle antenna apparatus is capable of, for example, receiving a circularly-polarized wave or a linearly-polarized wave sent from a satellite or a ground-based station. The radiation conductor has a patch-like structure, and an electric feeding point of the radiation conductor is connected with a central conductor of the coaxial cable. On the other hand, the ground conductor has a frame-like structure that surrounds the radiation conductor, and is separated from the radiation conductor by a predetermined distance. The ground conductor is connected with an outer conductor of the coaxial cable.
In an in-vehicle antenna apparatus of this type, an electronic circuit unit is attached to the inner surface of a sheet of glass, such as rear glass or front glass, facing the inside of the vehicle. This electronic circuit unit contains the circuit substrate which is electrically connected with the radiation conductor and the ground conductor disposed on the sheet of glass. This allows for the radiation conductor to receive electricity and load a received signal. In comparison with other types of antenna apparatuses that are set on the exterior of a vehicle, such as a roof, this type of antenna apparatus is advantageous in having a longer lifespan and a lower possibility of being stolen. Furthermore, in comparison with antenna apparatuses that are set in the vicinity of the inner surface of a windowpane of a vehicle, this type of antenna apparatus is advantageous in providing a good space factor and a wide angle of view.
In such a conventional in-vehicle antenna apparatus, however, the ground conductor surrounding the radiation conductor must be given at least a certain surface area in order to prevent the directional characteristics from deteriorating. This means that a ground conductor with a large dimension must be provided on the sheet of glass, such as rear glass or front glass, and is thus problematic in that such a large-size ground conductor may narrow the angle of view for vehicle occupants including a driver.
Furthermore, in this example of a conventional in-vehicle antenna apparatus, which is a flat patch antenna type having the radiation conductor and the ground conductor disposed on the inner surface of the sheet of glass, a radiation pattern (main lobe) mainly forms on the exterior of the sheet of glass. However, the antenna apparatus has directional characteristics in which a radiation pattern also forms slightly in the interior of the sheet of glass. This weakens the directional characteristics for intensely emitting radio-waves outward from the sheet of glass, and is thus problematic in that the radiation gain in the incoming direction of a tuned radio-wave becomes lower.